Larry,

> imagine that we assess overlap within L insula using a simple conjunction analysis, and
> find that there are 15 shared voxels between conditions A and B, along with 55 unique
> voxels for A and 5 unique voxels for B. Another form of our question could be what is the
> probability of observing this particular pair of unique voxel counts (55 and 5), again
> assuming no underlying difference between conditions?

Why do you consider those 55 (out of the 70 voxels under condition A) and 5 voxels (out of 20 voxels under condition B) "unique"? The two hypothetical clusters are identified per some arbitrary strength of statistical evidence (e.g., overall FWE of 0.05). Suppose there is a voxel within L insula but outside of the 70-voxel (or 50-voxel) cluster, you could say that the statistical evidence for the voxel is weaker than the preset comfortable level, but you cannot claim that there is no effect under condition A (or B): lack of strong evidence for an effect is not necessarily an evidence for lack of effect.

> What we would like to establish is something like a p value for the probability of observing
> clusters of these two sizes within the ROI by chance (i.e., assuming that there is no underlying
> difference between the two conditions).

I don't know the big picture, but why not directly making inference about the difference between the two conditions (e.g., condition A minus condition B) across the brain or within L insula?

Gang